This invention relates to thermocouples and more particularly to thermocouples for use in determining the temperature of molten metals.
Many kinds of thermocouples have been designed and used for use in the metallurgical industry. In general in the melting and casting processes for the production of primary and secondary aluminium the use of so called xe2x80x9cMarshall Tip Thermocouplexe2x80x9d has become fairly standard practice. In the baking process of carbon anodes for the production of aluminium the use of wire and bead or mineral insulated thermocouples protected by suitable metallic sheaths has become the norm.
In the ferrous metal industry platinum rhodium type thermocouples are used because the temperatures of molten steel are generally greater than those at which some of the components of the thermocouple used in the non-ferrous industry melt. It is difficult to provide insulation of the platinum rhodium element at molten steel temperatures for any length of time and insulation which will enable repeated use of the thermocouple is also difficult to provide. Consequently in the interests of economy, the thermocouple for this industry has been designed to protect the thermoelement for a maximum of about 4 seconds of immersion time, which is sufficient to obtain a single measurement.
Such thermocouples incorporate the smallest amount of the required materials, and where practical, the lowest cost materials in order to render the device expendable after only a single immersion into molten iron or steel.
Thus with an overriding cost consideration different thermocouples have been developed to meet the particular requirements of particular applications.
There remains however the basic requirements for all thermocouples which is the integrity of the temperature measurement obtained. To achieve this it is necessary that the measuring probe be protected against electrical conductivity of its immediate protection material and further that a barrier of sufficient mechanical strength be provided against the inherently corrosive attack from the in situ environment in which the thermocouple is to be used.
As stated the kind of thermocouple used in any application is driven by cost effectiveness. All of the thermocouples referred to suffer some or other disadvantage as a result of cost and it is the object of the present invention to provide a thermocouple which with minor modification can be used in the ferrous and non-ferrous industries and which can be made at a high cost effectiveness.
According to this invention there is provided a thermocouple comprising a sensing tip and electrical connection with a mineral insulated thermocouple cable characterised in that the shielding is provided by a low temperature sintering refractory material.
Further features of this invention provide for the thermocouple cable to be types K and N for non-ferrous metals or type W, W3, W5 and molybdenum rhenium for ferrous metals.
Still further features of this invention provide for the shielding to be in the form of a sheath having inner and outer metal tubes over a filler of low temperature sintering refractory material and for the tubes to be drawn down, swaged or rolled to compact the filler between them and for a conventional binder material to be added to the refractory material to give it the required green strength when the refractory material is beaded before introduction between the tubes.
The invention also provides for the refractory material to include particulate borosilicate and boric acid powder, for the borosilicate to comprise between 6% by weight of the total refractory material, for the boric acid to comprise about 3% to 5% by weight of the total refractory material and for the boric acid content of the refractory material to be about one half that of the borosilicate content.
Still further features of this invention provide for the inner and outer tubes of the sheath to be of stainless steel.
The invention provides a method of shielding a thermocouple comprising locating beads of suitably bound refractory material between an inner metal tube and an outer metal tube and reducing the sheath down to a predetermined size by drawing swaging or rolling during which process the beaded refractory material is compacted between the inner tube and the outer tube.
A further feature of this method provides for the reduced sheath to be subsequently annealed and the refractory material to be at least partially sintered simultaneously with the annealing of the sheath.
Yet further features of this invention provide for the tip to be provided by the dissimilar metal wires of the mineral insulated thermocouple cables providing a hot junction for the thermocouple with the wires embedded in magnesium oxide and this latter supported by a sheath as above defined or by a tube of the same metal as one wire of the cable housing the other wire of the cable to form the thermocouple tip with the wire embedded in a low sintering refractory material.
It is to be understood that where reference is made to metal tubes or wires of thermocouple cable materials being negative and positive Type K or Type W metals this polarity may be reversed. Further the terms xe2x80x9cshieldxe2x80x9d and xe2x80x9cshieldingxe2x80x9d are used to signify both thermal and electrical insulation.